Liquid crystals are now widespread as one of image display elements. Commonly adopted image display modes using liquid crystal materials include TN (twisted nematic) type display mode and G-H (guest-host) type display mode. Both of TN and G-H type display systems utilize nematic liquid crystals and, under the state of the art, show slower response as compared with other image display materials, such as a CRT, a plasma display, an electroluminescence display, etc.
Nevertheless, the image display system using liquid crystals is advantageous in that it is little fatigue of eyes and an electric power consumption is low. Further, with the recent increase of needs for large-sized and high-density display elements, it has been demanded to speed up the response of liquid crystal display elements which are suitable for constructing thin and light display devices. In recent years, ferroelectric liquid crystals have been developed as display materials having fast switching time, and image display devices taking advantage of their rapid electro-optical switching phenomenon have been proposed. Ferroelectric liquid crystals were reported in R.B. Meyer et al., J. de Phys. Lett., Vol. 36, 69 (1975), and they are considered to exhibit a chiral smectic C phase, a chiral smectic F phase, a chiral smectic I phase, a chiral smectic G phase, etc. (hereinafter referred to as S.sub.C * phase, S.sub.F * phase, S.sub.I * phase, or S.sub.G * phase, respectively) according to the liquid crystal classification. Of these phases, the S.sub.C * phase has the fast switching time and is believed to have the highest possibility of practical use in view of the phase structure and viscosity attributed to the phase structure.
In fact, N. A. Clark et al. observed an electrooptical switching phenomenon of microsecond order when pdecyloxybenzylidene-p'-amino-2-methylbutyl cinnamate (hereinafter abbreviated as DOBAMBC) that is one of the ferroelectric liquid crystal compounds and exhibits the S.sub.C* phase is sealed up within a very thin cell, as reported in Applied Phys. Lett., Vol. 36, 899 (1980) Studies on application of the ferroelectric liquid crystal (especially in the S.sub.C * phase thereof) to electroopticals, such as liquid crystal TV sets, photo-printer heads, nonlinear optical elements, and the like, taking advantage of such a high response rate, have already been taken up.
The response time .tau. of ferroelectric liquid crystals is represented by the following equation: ##EQU1## wherein P.sub.S represents a spontaneous polarization value; E represents an electric field; and .eta. represents a rotational viscosity (viscosity concerned in switching) [cf. Okano & Kobayashi, Ekisho, Kiso-hen, pp. 152-153, Baifu-kan (1985)].
As can be seen from this equation, spontaneous polarization has close relations with the response time, and it is considered that a liquid crystal compound having greater spontaneous polarization exhibits a higher response rate. However, spontaneous polarization of DOBAMBC is as small as about 3 nC/cm.sup.2. Therefore, introduction of various structural modifications have been suggested in order to attain larger spontaneous polarization. Since the very small spontaneous polarization of ferroelectric liquid crystals for the dipole moment possessed by themselves is believed to be ascribed to the influences of free rotation of a bond in the molecule, it has been proposed to introduce a structure in which free rotation between an optically active group (asymmetric carbon) and a chromophoric group is restricted, i.e., a structure in which an asymmetric carbon atom is adjacent to a chromophoric group. For example, Japanese Patent Application (OPI) No. 67453/85 (the term "OPI" as used herein means an "unexamined published Japanese patent application") discloses a structure of formula: ##STR2## which has a spontaneous polarization value of 17 nC/cm.sup.2, and Japanese Patent Application (OPI) No. 207486/86 discloses a structure of formula: ##STR3## which has a spontaneous polarization value of from 80 to 100 nC/cm.sup.2. Thus, liquid crystal compounds having these optically active groups solely composed of an alkyl group exhibit increased spontaneous polarization reaching about 100 nC/cm.sup.2.
Further, optically active groups having introduced thereinto a hetero atom (especially a halogen atom) have also been proposed. For example, the compounds having a structure of formula: ##STR4## wherein X represents a chlorine atom or a bromine atom, have spontaneous polarization values reaching 220 nC/cm.sup.2, as disclosed in Japanese Patent Application (OPI) No. 218358/85. These liquid crystal compounds show the highest spontaneous polarization among the so far developed ferroelectric liquid crystals (particularly S.sub.C* phase). However, in cases when such halogen-substituted compounds are used as display materials, they are regarded unstable optically and electrically and are, therefore, unsuitable for practical use.
In the light of the above-described circumstances, it has been keenly demanded to develop an optically and electrically stable liquid crystal compound having a spontaneous polarization value of 200 nC/cm.sup.2 or higher in its S.sub.C * phase.
Basic requirements for ferroelectric liquid crystals to have large spontaneous polarization values of 200 nC/cm.sup.2 or more and to be practically usable seem to be that an asymmetric carbon atom is close to a chromophoric group to form an optically and electrically stable structure; that there is a dipole moment having a vector component perpendicular to major axis of the molecule; and that the liquid crystals show an S.sub.C * phase. In pursuit of liquid crystal compounds satisfying these requirements, a number of compounds have now been developed. However, as mentioned above, there has been developed only one type of liquid crystal compound having a large spontaneous polarization value of 200 nC/cm.sup.2 or higher, in which a hetero atom, such as a halogen atom, is introduced to the asymmetric carbon.